Fuel injectors are used to deliver fuel under high pressure to a combustion space of an engine. It is known to use multi-hole fuel injection nozzles in internal combustion engines, such as diesel engines with direct injection diesel combustion systems. One such multi-hole fuel injection nozzle is disclosed in European Patent No. 1626173 and comprises a nozzle body having concentric valve needles to control the flow of fuel through respective upper and lower spray holes.
Due to increasingly stringent engine emissions regulations, it is highly desirable to reduce diesel engine exhaust soot emission by optimising the mixing of air and fuel within the engine combustion chamber. To this end, European Patent No. 1059437 describes a multi-hole injection nozzle which provides improved optimisation of air/fuel mixing. FIG. 1 shows an injection nozzle of the kind described in EP-1059437.
Referring to FIG. 1, the injection nozzle comprises a nozzle body 1 having a blind bore 2. The nozzle body 1 defines a primary nozzle axis A-A which is co-axial with the blind bore 2. The blind end of the bore 2 is provided with upper and lower spray holes 3, 4 and defines a seating with which a valve needle (not shown) is engageable to control the supply of fuel to the upper and lower spray holes 3, 4. The valve needle may be of the type described in EP-1626173, where inner and outer concentric valve needles are provided to control the flow of fuel through both the upper and lower spray holes together or through the upper spray hole only.
When fuel flows through both the upper and lower spray holes 3, 4 together, first and second fuel sprays are emitted having axes labelled 7 and 8, respectively. The first and second fuel sprays 7, 8 merge to form a single fuel spray jet which gives the effect of a single large spray hole in terms of fuel mass flow and penetration of the fuel spray jet.
The intersection point 6 of the first and second fuel sprays 7, 8 lies a distance RC into the combustion chamber, in a direction perpendicular to the primary nozzle axis A-A. The line 9 defines the axis and direction of the merged fuel spray jet. The vertical distance X3 below the flame face 10 of the engine cylinder head (i.e. at the ceiling of the combustion chamber) at the radius RC from the centre of the combustion chamber gives a vertical target direction of the merged fuel jet.
At high engine loads and speeds with injection from both spray holes 3, 4, the vertical target of the merged fuel spray jet 9 corresponds to the distance X3 below the flame face 10 of the engine cylinder head at the radius RC from the centre of the combustion chamber.
At low loads and speeds, with injection from only the upper spray hole 3, the vertical target of the fuel spray jet from the single spray hole also corresponds to the distance X3 below the flame face 10 at the radius RC from the centre of the combustion chamber.
In the case that the injection nozzle of FIG. 1 is provided with a valve needle which is operable, at low engine speeds and loads, to inject fuel through only the lower spray hole 4, the vertical target of the fuel spray jet from the single spray hole also corresponds to the distance X3 below the flame face 10 at the radius RC from the centre of the combustion chamber.
Accordingly, with the conventional injection nozzle of FIG. 1, regardless of whether fuel is injected through only one of the spray holes 3, 4 or through both of them together, the vertical target distance of the resulting fuel spray jet is the same. This is problematic for obtaining the lowest possible soot emissions because the vertical target distance which is optimal for air/fuel mixing is known to vary as a function of the engine speed/load.
It is an object of the present invention to provide a fuel injection nozzle which substantially overcomes or mitigates the aforementioned problem.